Multi-effect Evaporators: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • Using multiple evaporators in series decreases the amount of energy needed to evaporate 1 kg of liquid (more than 3 evaporators can be used).
  • Steam economy [(mass flow rate of vapor leaving system)/(mass flow rate of steam fed to system)] is often between 2 and 3 for a triple-effect evaporator.
  • The heat transfer is often set to be the same in each effect by picking the pressure for each effect (and the corresponding temperature).
  • The last effect in a multi-effect evaporator is often below atmospheric pressure.
  • Mass and energy balances can be used to determine how much the feed is concentrated in each effect.
  • The pressure (and thus the boiling point temperature) is highest in the first effect and lowest in the third effect of a triple-effect evaporator.
  • As solute concentration increases, the boiling point increases.
  • For feed-forward, multi-effect evaporators. the feed enters the first effect, which is at the highest temperature. For backward feed, the feed is fed to the last effect.
  • For backward feed to multi-effect evaporators, pumps are needed to transfer the liquid from one effect to another because the pressure increases from the last effect to the first. 
  • Backward feed might be used if the feed is cold or if the feed becomes viscous so that the more viscous solution is at the higher temperature.
From studying this module, you should now be able to:
  • Solve mass and energy balances for a multi-effect evaporator.
  • Explain why pressure decreases from the first effect to the second effect.
  • Calculate steam economy for a multi-effect evaporator.
Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder